A transmitting method of large capacity data using a digital coherent transmission technology has been researched and developed. As the digital coherent transmission technology, for example, a multi-level modulation such as quadrature amplitude modulation (QAM), or the like is used.
Further, with respect to this type of transmission technology, an encryption method using the QAM has been proposed (see, e.g., Patent Documents 1 and 2 and Non-Patent Documents 1 and 2).
Related technologies are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2014-093764 (Patent Document 1) and Japanese National Publication of International Patent Application No. 2008-514159 (Patent Document 2).
Related technologies are also disclosed in, for example, Masataka Nakazawa et al., “QAM quantum stream cipher using digital coherent optical transmission,” IEICE, QIT 2014-57, November, 2014 (Non-Patent Document 1) and Masataka Nakazawa et al., “QAM quantum stream cipher using digital coherent optical transmission,” OPTICS EXPRESS Vol. 22, pp. 4098-4107, Feb. 24, 2014 (Non-Patent Document 2).